FIELD OF THE INVENTION
The invention relates to a data transmission apparatus with a subscriber connection for a telephone line of a telephone network, a data connection for connection to a computer, a demodulator section for converting an input signal received from the telephone line into digital data which are transmitted to the computer, and a signal detection device for detecting a special signal contained in the received signal at times, which special signal is produced by a telephone network exchange.
Data transmission apparatuses by means of which a computer can be connected via a conventional telephone line to another computer are normally called modems. Such modems are frequently used in conjunction with PCs. The modems render it possible for the PC to set up a connection via a conventional telephone network to another computer or to a network of computers. That "host" computer or network is also connected to the telephone network. It is not rare for such connections between a user's PC and a data network, such as the Internet, to last over lengthy time periods. Due to the fact that PC users--particularly in private use--often have only one telephone line that is used both for conventional voice communication and for data communication through the computer modem, the user is not available for incoming voice calls while a data session is taking place.
U.S. Pat. No. 5,493,609 to Winseck, Jr. et al. discloses a telecommunication system for automatically switching between voice and data communications. There, dynamic switching is possible on a conventional telephone line, between a voice mode and a multiplicity of data modes. In the basic mode, voice operation takes place between two subscribers. When one subscriber sends a start signal, the voice operation is ended and the system changes over to data mode.
Modern telephone switching systems offer the telephone subscriber a "call waiting" feature. If the feature is activated, an audible signal is inserted for the subscriber while the telephone call is taking place. The signal indicates to the subscriber that a further call has been received at the exchange. The subscriber then has the option of signaling to the exchange whether he wishes to terminate or interrupt the existing call in order to receive the waiting call ("flash"), or whether he wishes to continue with the current call. The presence of a waiting call is indicated to the subscriber by adding an audio signal to the call signal coming from the exchange, which the subscriber receives acoustically while the call is taking place.
If the existing connection is a data communication through a modem, then it is not possible for the PC user to perceive the "audible" call-waiting signal in the incoming signal sent from the exchange to the modem. In fact, there is even a risk that the call-waiting signal added to the received signal may cause errors in the demodulation of the received signal. Often, this leads to the termination of the data connection by the driver software or by the communications software driving the modem. There is thus a desire by data communications users to be able to receive a telephone call even while a data communication is taking place, without the data communication being unnecessarily adversely affected by this.
U.S. Pat. No. 5,519,767 to O'Horo et al. discloses a voice and data modem with the "call waiting" function. While that modem can identify a call waiting signal, it ignores the signal in the subsequent data processing in the modem.